1. Field of the Invention
The present invention relates to a phase shift mask used in the photolithography process for manufacturing a semiconductor integrated circuit device, and a method of fabricating the phase shift mask.
2. Description of the Related Art
Conventionally, in a mask used in the photolithography process which is one of the major processes of manufacturing a semiconductor integrated circuit device, a desired pattern is formed of light-shielding materials, such as chromium and tungsten, on a light transmitting substrate of quartz or the like. The intensity of light transmitted through this type of mask does not change suddenly because of scattering or interference of the light at the edges of the pattern, which causes a drawback of preventing a very small pattern from being resolved.
To eliminate the above drawback, an attempt to improve the resolution of a pattern has been made by reducing the light interference effect by shifting a phase of part of the transmitted light. This attempt is called a phase shift method, and a mask formed by the method is called a phase shift mask. The phase shift method includes various types of methods, such as the Levenson method, auxiliary pattern method, edge emphasis method, multistage method, chrome-less method, and shifter light-shielding method. The phase shift mask is provided with a phase shifter as component for shifting the phase of light for exposure. For example, photoresist can be used to form a phase shifter, or a projection of that uneven surface of a glass substrate which is obtained by etching can be used as a phase shifter. This type of phase shifter is described in Nakagawa et al., Nikkei Microdevices, "Phase Shift Exposure Development of Shifter Light-Shielding Method Prioritizing Easiness of Use," May, 1991, pp. 53-58.
The mask having a phase shifter formed of photoresist has the advantage of allowing the phase shifter to be self-aligned with a chrome film (light-shielding film pattern), and can be formed relatively easily, finely, precisely. Since, however, the photoresist of the phase shifter is an organic film, the finished mask cannot resist good (or strong) washing, e.g., washing using acid, and has the disadvantage of easily causing a flaw in the surface of the mask.
In contrast, the mask having a phase shifter formed by etching a glass substrate is able to overcome the above disadvantage of the phase shifter formed of photoresist. Since, however, there are no members for stopping the etching of the glass substrate, the etching varies in depth, thus making it difficult to form the phase shifter.
A method of fabricating a phase shifter by SOG (Spin on Glass) has recently been proposed and brought into the public eye. This method is described in Nikkei Microdevices, "Investigation of SOG, Sputtering and CVD," 1991, May, p. 64.
Organic matter having a methyl group is used for the SOG since it is increased in fluidity and easily applied to a substrate, a film, and the like. Since, however, the organic matter causes damage to the transparence (transmittance) of a film, light is attenuated when penetrating the phase shifter. Since the organic matter is evaporated when the phase shifter is baked, the phase shifter is decreased in volume and thus difficult to form according to the specifications. Furthermore, since the glass substrate is rotated in fabricating the phase shifter, SOG is caused to flow on, e.g., a chrome film (light-shielding film pattern) by the centrifugal force, the SOG is formed thick in the recess portion and as is formed thin in the projection surface. It is thus difficult to uniform the thickness of the SOG and to determine the size of the phase shifter.